1. Field of the Invention:
This invention relates to a free piston type auto-tensioner for adjusting the tension of an engine timing belt in apparatus such as an automobile.
2. Description of the Related Art:
Various types of auto-tensioners for preventing the slackening of an engine timing belt for automobiles have been proposed. One example of a prior art auto-tensioner is disclosed in the Japanese patent application laid-open publication No. 59(1984)-126144 as shown herein at FIG. 4. In FIG. 4, an auto-tensioner comprises a body 1 in which a plunger 2 is slidably disposed so as to form a large diameter oil chamber 4 having a seal member 3. The plunger is continuously biased in the direction of the oil chamber 4 by a spring 7 interposed between a snap ring 5 and a stepped portion 6 of the plunger 2.
The plunger 2 is provided with a bore 8 in which a piston 9 is slidably disposed so as to form a small diameter oil chamber 11 communicating with the large diameter oil chamber 4 through a small diameter passage 10. The small diameter oil chamber 11 is sealed from the outside by means of a seal member 12. Secured to an open portion of the bore 8 is a snap ring 13 which may come into contact with a stepped portion 14 of the piston 9 in order to prevent the piston 9 from coming out. A head portion 9a of the piston 9 protrudes outwardly from the body 1. An end surface 9b functions as a pushing surface of the auto-tensioner.
FIG. 3 generally shows an auto-tensioner c installed on a bracket b, and which pushes a belt a via an idler pulley d to tension the belt. When an engine temperature rises from a normal temperature to a high temperature, thermal expansion causes an increase in the distance between a cam and a crank whereby the tension of the belt a shown in FIG. 3 is increased. At this time, since the load applied to the piston 9 of the auto-tensioner c is increased, the oil pressure within the small diameter oil chamber 11 is also increased. Therefore, the oil flows from the small diameter oil chamber 11 into the large diameter oil chamber 4 through the small diameter passage 10 with the result that the piston 9 is moved downward within the body 1 and that the increase in the belt tension is absorbed.
On the other hand, when the engine temperature falls from the high temperature to the normal temperature, the tension of the belt a is decreased whereby the load applied to the piston 9 is decreased. Therefore, due to the oil pressure within the small diameter oil chamber 11, the piston 9 protrudes from the body 1 and pushes the belt, whereby the decreased belt tension can be absorbed. Thus, an appropriate tension of the belt can be maintained.
In the above prior art auto-tensioner, however, when the piston 9 is pushed down due to increased belt tension, a high oil pressure is generated in the small diameter oil chamber 11. Since the high oil pressure acts on the seal member 12 through a gap between the piston 9 and the plunger 2, the seal member 12 is indesirably deformed.
Furthermore, the high oil pressure in the small diameter oil chamber 11 acts on the large diameter oil chamber 4 through the small diameter passage 10. Since the large diameter oil chamber 4 receives the biasing force of the spring 7, the high oil pressure in the large diameter oil chamber 4 has a bad influence on the seal member 3 via the gap between the plunger 2 and the body 1. Therefore, the sealing function of the seal members 3 and 12 is deteriorated and the oil eventually leaks past the seal members 3 and 12.